The term "kine" as used herein means the same as television picture tube.
Black level drift has long been a problem in television recievers. It is characterized by an undesirable variation in the visual black level or brightness of the displayed picture during warm-up of the television receiver. The primary causes of black level drift are parameter variations caused by temperature changes in the receiver circuitry and in the kine itself.
Electronic circuits employing semiconductors are subject to operating parameter changes in response to temperature variations in and around the electronic components of the circuits. The temperature variations are caused by self-heating of the components and by a rise in ambient temperature within the cabinet caused by warm-up of other components.
The self-heating phenomenon occurs rather quickly, that is, within five minutes of application of power to, for example, a television receiver. On the other hand, the ambient temperature rise occurs relatively slowly over a period which may be ten times as long.
As the kine itself warms up, the control grid (G1) physically moves with respect to the cathodes, causing a change in the kine cutoff characteristics which also varies the black level.
Modern television receivers are employing multifunction integrated circuit chips (ICs) with heretofore unprecedented levels of complexity and circuit density. In order to reduce the power dissipation of these multifunction ICs it is common practice to limit the output signal swing peak-to-peak. The peak-to-peak signal at the input of the kine driver circuitry is thus at a lower level than was common in previous television receivers, and a higher gain kine driver amplifier is therefore required to produce the same level of brightness on the kine screen as produced by previous television receivers.
A recent trend among television manufacturers has been to provide kine face plates with a lower light transmissivity (52% rather than 85%) in an effort to improve picture contrast. This trend has caused the gain of the kine driver amplifier to be even further increased to provide a suitably bright picture. Typically the gain of the kine driver amplifier has been increased from the previous level of 40 to the present level of 60.
As a result of employing these high gain kine driver amplifiers a new problem has arisen, in that gain and/or offset variations of the video signal processing circuitry caused by temperature variations during warmup which previously may have been at an acceptable low level, are now aggravated to an objectionable level. This magnitude variation may manifest itself as an observable change in the black level on the kine screen.
It is known from U.S. Pat. No. 4,716,315 (Bell) to compensate for temperature variations in a video peaking circuit of a television receiver by adding a thermistor to compensate for the long-term ambient effects, and by adding a separate gain control circuit having a time constant approximating the relatively quick self-heating time to compensate for self-heating-caused parameter changes.